if I create ‘front-page.php’, then how do I link to post index?

Since WordPress 4.5 you can use get_post_type_archive_link( 'post' ) to link to the page containing the blog posts. Depending on what you’ve set under Settings -> Reading, this will be either the front page, e.g. example.com/ or a specific page like example.com/news/. home_url( "https://wordpress.stackexchange.com/" ) always points to the front page.